An avalanche photodiode (APD) is a sensitive type of optical sensor, which converts optical radiation into electric current, which is proportional to the optical power. The APD is connected on one side to a high-voltage supply and on the other side to a load. Conversion of the optical radiation into current by the APD uses energy from the DC high-voltage supply. The DC supply provides enough energy for the APD to generate pulses of high current, wherein the maximum current can reach 10 A. Such overcurrent may occur, for example, if the APD is exposed to a bright light, and may damage the APD or the load.
The problem of excessive APD current is known in the art. For example, PCT International Publication WO 2010/020278, whose disclosure is incorporated herein by reference, describes an APD circuit, comprising an APD, typically connected in parallel to a capacitor, with a current shunt circuit. The current shunt circuit activates to shunt current from the APD in response to detecting a decrease in the impedance of the APD, which is typically measured by determining the slope of the voltage across the APD. By using this circuit, the APD is said to be protected from sudden increases in incident light level decreasing the impedance of the APD to an extent that the energy such as is stored in the capacitor can damage the structure of the APD.